The present invention is directed to an all-optical flip-flop device for optical communication networks and more specifically to such a device having all-optical set and reset capabilities.
According to the Photonics Spectra May 2000 issue, the global optical networking market totaled $3.9 billion in 1999 and will grow in 2004 to $17 billion, of which $15 billion will be in long-haul networks and $2 billion in metropolitan systems. That phenomenal growth reflects the demands of the Internet and is expected to continue to be accompanied by a high investment in new technology.
Optical communication networks have a need to convert the short pulses sent on backbone and trunk lines at 10 Gb/sec or faster to the longer pulses used on local networks. Such networks often also have to convert frequencies, since it is often advantageous to operate local networks on a different frequency from that used for backbone and trunk lines. Such conversion can now be done on wavelength division multiplexed (WDM) systems by converting the optical signals to electronic signals and converting those electronic signals back to optical signals by way of a photodetector, an electronic decision circuit and an output laser. Pulse length conversion is not done routinely in communication networks, because the existing technology is expensive.
Various options are being considered for all-optical switches. One such option is MEMS (micro-mechanical systems), offering a switch time of 10-50 msec. Others are liquid crystal technology, offering a switch time of 4-10 msec, and bubble-jet technology, offering a switch time of 10 msec.
The bistable behavior of optical signals propagating through resonant-type semiconductor optical amplifiers (SOAs) has been used to demonstrate optical switching. Those devices are useful for lightwave systems because they are compact, can be fabricated at any wavelength, and exhibit inherent amplification and therefore high fan-out and high cascadability. Switching in such devices has been measured at microwatt power levels. Thus, those devices easily operate at power levels available in lightwave systems. Since typical switching speeds are on the order of 1 ns, switching occurs at femtojoule energies (xcx9c7000 photons).
Another developing area in optical computing and communication systems is optical memory. All-optical digital memory promises many advantages for optical communication systems. Compared to electronic digital memory, the optical domain promises faster speeds of greater than 10 Gb/s. Furthermore, the above-noted necessity for optical-to-electronic and electronic-to-optical conversion is avoided, which would allow the memory function of a optical network node to be integrated with other photonic functions, such as wavelength conversion and routing.
Optical memory has been demonstrated using high-speed fiber loops, but that method supports only analog formats, exhibits fixed memory durations, and is typically bulky. In contrast, optical memory exhibited by bistable semiconductor lasers offers digital operation within a compact device. Flip-flop operation based on polarization bistability in semiconductor lasers is expected to be ultrafast (xcx9c100 Gb/s), but requires orthogonally polarized control pulses that would be expensive to maintain in a fiber-optic system. Diode lasers with integrated absorption regions have been used at submilliwatt powers over a 28-nm spectral range, but it is difficult to perform both set and reset optically. Flip-flop operation can also occur with a holding beam undergoing dispersive bistability within a resonant-type semiconductor optical amplifier (SOA), but those previous control techniques have a very limited wavelength range; set and reset occur either by varying the holding-beam input power or by modulating the holding beam with a closely tuned (0.008 nm) auxiliary signal.
It will be apparent from the above that a need exists in the art for an optical switch, memory or other device which overcomes the above-noted difficulties.
It is therefore an object of the invention to provide a bistable optical device which can be switched between its stable states using temporally short pulses.
It is another object of the invention to provide a bistable optical device which can be switched between its stable states with low power consumption.
It is yet another object of the invention to provide such an optical switch, memory or other device which can be switched optically, so that no optical-to-electronic conversion is required.
It is yet another object of the invention to provide such an optical switch, memory or other device which can operate with control signals over a wide wavelength range.
It is yet another object of the invention to provide all-optical processing at data rates beyond those inexpensively accessible by electronic processing by providing such an optical switch, memory or other device which can operate using pulses shorter than 100 ps.
It is yet another object of the invention to provide such an optical switch, memory or other device which can operate at low power levels ( less than 1 mW) over a wide and flexible wavelength range, and which can be transparent to the control signals"" polarization.
It is yet another object of the invention to provide such an optical memory which can store data digitally.
It is yet another object of the invention to provide such an optical switch, memory or other device which is compact.
It is yet another object of the invention to provide such an optical switch, memory or other device which latches to a desired state.
To achieve the above and other objects, the present invention is directed to a bistable device whose holding beam is set and reset by varying the hysteresis itself. As the hysteresis varies, so does the transition between the two stable states, so that the device goes back and forth between the two stable states with a constant holding beam input power. The hysteresis is typically changed by applying one or more optical signals, such as separate set and reset pulses, to change the gain and therefore the refractive index of the bistable device. The set pulse or reset signal can be modulated in accordance with data, or a data signal from another optical device can be directly used as a set pulse or reset signal.
The holding beam can operate at microwatt power levels and experiences amplification. Once switched to the upper state, the output power of the holding beam remains high until reset. Thus, both an optical switch and an optical memory, as well as other uses, can be implemented.
The small size and integratability of SOAs allow, in principle, the fabrication of monolithic optical buffers. Such devices would be more compact than fiber-delay lines. For example, an array of 32 amplifiers can be used for storing the packet header required for routing signals through a crossconnect optical circuit.
The present invention provides all-optical mechanisms for such setting and resetting over wide wavelength ranges.
The latching capability of an optical flip-flop allows the output to be maintained for processing at a later time. A digital, sequential means of processing is thus available for applications such as bit-length conversion, data-format change, demultiplexing, packet-header buffering and retiming.
The present invention provides robust optical techniques for controlling a resonant-type-SOA-based optical flip-flop. Set and reset are performed by XPM (cross-phase modulation), rather than by changing the input power of the holding beam. Control signals operate at submilliwatt powers over wide wavelength ranges that intersect important communication bands centered near 1310 and 1550 nm. XPM is transparent to the direction of incidence, and reset is independent of polarization. The set-signal polarization dependence can be eliminated, and repetition rates greater than 10 Gb/s can be achieved. The latching capability of such a fast, robust all-optical flip-flop will significantly advance the development of all-optical digital processing within communication systems.
The present invention, because of its latching capability, can increase the pulse length (increasing the energy in the pulse), while it simultaneously performs a wavelength conversion. In addition to the capability for pulse length conversion, the latching capability holds the signal in memory and provides the opportunity to change the data format with the device. The present invention has still further utility as an optical memory with a digital response with pulses shorter than 100 ps.